%I #10 Oct 08 2023 04:53:02
%S 3,4,6,8,9,12,15,16,18,20,21,24,25,27,28,30,32,33,36,39,40,42,44,45,
%T 48,49,50,51,52,54,56,57,60,63,64,66,68,69,72,75,76,78,80,81,84,87,88,
%U 90,92,93,96,98,99,100,102,104,105,108,111,112,114,116,117,120,121,123,124,125,126,128,129,132,135,136,138
%N Numbers k such that 3*k is not squarefree.
%C The asymptotic density of this sequence is 1 - 9/(2*Pi^2). - _Amiram Eldar_, Oct 08 2023
%t Select[Range[140], !SquareFreeQ[3*#] &] (* _Amiram Eldar_, Oct 08 2023 *)
%o (PARI) isA366288(n) = !issquarefree(3*n);
%Y Complement of A261034.
%Y Union of A013929 and A008585 (after its initial 0).
%Y Positions of zeros in A156277, in A359377, and in A365428.
%Y Cf. A366290 (subsequence).
%K nonn,easy
%O 1,1
%A _Antti Karttunen_, Oct 07 2023